The use of radio communication systems to provide telephony service has been seen to provide several advantages over conventional wireline networks. These advantages include speed of deployment, lower cost of installation, and reduced maintenance of outside plant. In a fixed wireless access system, base stations, which include a radio transceiver and which communicate with a telephone network, are placed at intervals within a neighborhood so as to be within range of subscriber units. A subscriber unit (SU) is a user device which contains a radio transceiver for communicating to a base station.
In many cases, the radio system used as the basis for a fixed wireless access system is one originally designed for mobile cellular telephony. In cellular-type systems, there is a network element, known as a mobile telephone switching office or a mobile switching center (MSC), which acts as an interface between the radio equipment and the telephone network (PSTN). The MSC performs many functions, including the receipt of dialed directory numbers, analysis of dialed directory numbers, activation of services or routing of calls to voice/data facilities, e.g. trunk or line facilities, and billing.
When a subscriber originates the call using a conventional mobile cellular subscriber terminal, the digits entered by the user are collected and stored locally in the terminal until the user presses a "SEND" key to indicate that the entire set of digits has been entered. At that point, a radio signaling link to a base station is used to transmit the entire set of entered digits in a message to the base station. The base station then transmits the entered digits, typically a directory number, to the MSC. The MSC receives the set of dialed digits representing a directory number entered by the subscriber as a group. Alternatively, the user may not be originating a call, but rather, may be requesting a feature or service (e.g. call forwarding), in which case, the digits received by the MSC would be a feature activation code. Upon translating these digits, the MSC determines whether a voice/data facility is required, selects the voice/data facility, takes the necessary steps to set up a call on the outgoing facility and connects the subscriber's voice/data facility to the outgoing facility. Typically, the MSC uses the dialed digits, the corresponding directory number and other information related to the subscriber for such purposes as billing the subscriber for the call.
Due to the cost differential in producing mobile cellular terminals as compared to conventional telephones, and due to the fact that a fixed wireless access terminal does not require mobility, it has been proposed to utilize conventional telephones attached, by means of a suitable interface, to a cellular transceiver as the subscriber unit (SU). However, when there is a fixed access SU controlled by a wireline telephone set, a problem arises: there is no "SEND" key on a wireline telephone set to indicate explicitly the end of dialing; i.e. when an entire valid called-party directory number or when an activation code requesting a particular service or feature (hereinafter referred to as a feature activation code) has been entered.
One solution to this problem was disclosed in U.S. Pat. No. 4,658,096 by West, Jr. et al. This patent teaches the use of a SU which provides standard wireline telephony signaling to the telephone set, such as ringing voltage, and supplies direct current for the telephone loop. The SU also contains means for detecting digits dialed or entered by the user on the telephone, which are produced by the telephone in the formats used in analog telephony (loop disconnect (sometimes called dial pulse) or dual-tone multi-frequency (DTMF) signaling), and means for converting the detected digits into digital representations of these digits which are used in the cellular telephony system. These digits are then stored in a conventional cellular radio transceiver. This system includes a means for determining when the last digit of a valid called-party directory number had been dialed, at which point the entire called-party directory number is sent by the radio transceiver to the base station. The means for determining when the last digit had been entered involved a combination of analysis of what digits had been dialed and the number of digits entered. Essentially, knowledge of the North American dialing plan was built into the SU in order for the SU to determine when a complete valid directory number had been entered. In the system of West, Jr. et al, during the time period in which the calling subscriber had placed the telephone off-hook, but had not yet entered any digits, a radio link to a base station did not exist. In order to emulate the user interface of a conventional wireline telephone network, the interface locally produced a dial tone which the subscriber would hear in the telephone set's receiver.
A disadvantage of this solution is that the SU requires extensive electronic circuit hardware or alternatively, micro-controller memory, to perform the digit analysis. This can increase the cost and size of the SU. Also this complicates manufacture, as these units will not be easily mass produced for export, as SUs will need to be configured to suit the dialing plan for each country the units are sold in. In addition, if there are changes to the dialing plan of the territory where the system is deployed, then the SUs already deployed may not be able to originate calls properly, and may have to be withdrawn from service.
Accordingly, there is a need for collecting dialed digits in a fixed wireless access system which does not require building into the SU the knowledge of the dialing plan of the PSTN in the territory of deployment.